Abnormal Threshold Voltage Shift of Amorphous InGaZnO Thin-Film Transistors Due to Mobile Sodium

The negative bias stress normally yields a negative threshold voltage shift of the thin film transistors due to the additional positive charges trapped in the gate dielectrics or at channel/gate insulator interface. However, a positive threshold voltage shift of the device with the post InGaZnO deposition annealing at 400 °C is observed in our devices. The Na
⁺
incorporation from Mo gate into the gate dielectric after 400 °C annealing is responsible for this abnormal threshold voltage shift. The movement of Na
⁺
ions toward the gate electrode by the negative gate bias decreases the distance between the gate electrode and the Na
⁺
ions. Therefore, the voltage drop between the gate electrode and the Na
⁺
ions reduces, and a corresponding positive threshold voltage shift is observed. Inserting a SiN
_x
layer between the SiO
_x
gate insulator and the Mo gate electrode can reduce the Na
⁺
mobility, and thus a normal negative threshold voltage shift resumes.